This invention is particularly but not exclusively concerned with the type of printing apparatus which are known as label printers. These apparatus have a housing carrying a data input device in the form of a keyboard for inputting a message to be printed and containing a printing mechanism and a drive system. The housing also includes a cassette receiving bay for receiving a cassette which houses printing tape and ink ribbon. There are several different types of this kind of label printer. Some receive a single cassette which houses at least a printing tape and an ink ribbon, as described for example in our earlier European Application No. 91310664.7. In another system, the cassette receiving bay receives two separate cassettes, one housing an ink ribbon and the other housing a printing tape. Such a system is described for example in our earlier European Application No. 93303971.1. The contents of these earlier applications are incorporated herein by reference. In these and other known types of systems, the ink ribbon and printing tape are passed in overlap between a thermal printhead and a platen of the printing mechanism. For printing, the ink ribbon is pressed against the printing tape between the thermal printhead and the platen and pixel data to be printed is passed to the thermal printhead. Normally, the thermal printhead comprises a column of printing elements to which data is supplied and printed sequentially. During printing, the printing tape is driven through the printing zone defined by the thermal printhead and platen so that adjacent columns are printed sequentially in the direction of movement of the printing tape, thereby forming characters etc to be printed.
As described in the above-referenced European Applications, the printing tape is a multilayer printing tape having an image receiving layer and a backing layer secured to the image receiving layer by an adhesive layer. The label printer includes a cutting mechanism for cutting off a portion of the multilayer tape after printing to form a label. The backing layer of the label can then be removed to allow the label to be stuck to any object.
FIG. 1 illustrates the elements of a drive system of a known printing apparatus. Reference numerals 2 and 4 denote the platen and thermal printhead respectively which constitute the main components of the print mechanism. Reference numerals 6 and 7 denote the tape 6 and ink ribbon 7 which are passed in overlap between the platen and thermal printhead for printing. Although not shown in FIG. 1, the ink ribbon lies adjacent the thermal printhead 4 and is wound from a supply reel to a take-up reel, normally within a cassette. In practice, the ink ribbon is driven past the printhead by the action of friction between the printing tape and the ink ribbon, the two being intended to run together at the same speed. The take-up reel is driven so that if free to do so it would pull the ink ribbon past the printhead faster than the platen would. A slipping clutch is normally provided to ensure that the ink ribbon moves at a speed defined by the platen motion, the clutch ensuring that ink ribbon slack is always taken up and tension maintained. The take-up reel can be driven with the platen to ensure that the ink ribbon is wound up, but other drive arrangements are possible. The platen is in any event driven to rotate and presses against the printing tape on one surface thereof, the other surface lying against the ink ribbon. The tape 6 is thus driven past the thermal printhead 4 by the action of friction between the tape 6 and the platen 2, which is normally made of rubber. The platen 2 is driven by a stepper motor 8 through a gear train which is illustrated only diagrammatically at reference numeral 10. The stepper motor 8 is in turn driven by signals from a microprocessor 12 via a driver chip 14. The stepper motor 8 completes a rotation in a number of discrete steps in response to a series of pulses sent from the microcontroller, normally one step per pulse. Reference numeral 15 denotes a power supply for the motor and microprocessor.
At the same time, data for printing is sent to the thermal printhead 4 from the microcontroller via the data line 16. The thermal printhead includes a shift register and a separate parallel storage register. Data is transferred to the printhead serially, clocked bit by bit under the control of the microcontroller into the shift register contained in the printhead assembly. At the end of the transfer of a column of pixel data, the data is latched into the storage register under command from the microcontroller. The storage register will hold this data until the next latching operation of new shift register contents into the storage register. Later, the printhead is "strobed" by the microcontroller to turn on high current output drivers in parallel which deposit melted ink from the ink ribbon onto the tape 6 in pixel patterns according to data held in the storage register. Clocking of data into the shift register can be occurring while a strobe signal causes printing of the data in the storage register, but it is not necessary that the operation occurs in this way, since the operations are essentially independent. As explained above, the thermal printhead has a column of printing element which are printed as a vertical line on the printing tape. A character is thus printed by printing a number of adjacent and slightly overlapping columns containing different pixel data on the printing tape as it moves past the thermal printhead.
The microcontroller 12 arranges for a certain number of step pulses to be sent to the stepper motor for each print column strobe signal, in a defined sequence in order to produce the correct relationship between tape motion and print data, thereby forming correctly proportioned characters. This process is continuous only in the sense that the motor rotates step-wise at roughly a constant speed during printing of a label. The motor does not continuously rotate.
The reference clock for the step pulses and for the print strobe signals is the same, derived for example from a crystal oscillator 18. The system relies on the assumption that, once the microcontroller has sent out the correct stepper motor drive and strobe signals in response to the reference clock, the motor and tape move as expected while ink is deposited on the printing tape at the thermal printhead. However, circumstances can arise where the tape can jam as a result of high friction levels at the platen, or elsewhere, such as in the gear trains. In that case, the motor will cease to step and adjacent columns of pixels will be printed overlapping one another, resulting in a useless label.
Moreover, a stepper motor is a relatively expensive component of label printers and has a relatively high power requirement. This is particularly disadvantageous where the label printer is to be operated on batteries.